Published on Web 04/01/2006
Highly Reversible and Multi-Stage Cantilever Actuation Driven by Polyelectrolyte Brushes Feng Zhou,†,‡ Wenmiao Shu,† Mark E. Welland,† and Wilhelm T S Huck*,†,‡ The Nanoscience Centre, UniVersity of Cambridge, 11 J.J. Thomson AVenue, Cambridge CB3 0FF, U.K., and MelVille Laboratory for Polymer Synthesis, Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K. Received January 27, 2006; E-mail:
[email protected] The bending of microcantilevers upon adsorption of polymers (DNA, proteins) or small molecules has great potential for the development of highly sensitive sensors and efficient nanoactuators.1,2 For microcantilevers to be useful as actuators, precise positioning, reversibility, and large-scale bending are prerequisites.3 Conventional modification by self-assembled monolayers (SAMs) usually generates small cantilever deflections.4 By grafting polymers to the cantilever surface, a much wider range of responses can be achieved due to conformational changes in the polymer backbones,5-7 and recently, the bending of pH responsive copolymer brush-coated AFM cantilevers was studied under different conditions.8 However, reversible and multi-stage actuation of cantilevers remains a significant challenge. The use of polyelectrolytes and their collapse in response to salt has recently9 emerged as a promising potential synthetic equivalent of one of the most powerful biological motors: the spasmoneme spring.10 In this communication, we report the first example of polyelectrolyte brush-driven highly reversible actuation of cantilevers, expanding the range of triggers leading to mechanical actuation as well as providing new fundamental insights into polyelectrolyte brushes. The brushes are selectively grown on one side of the cantilever via a sequential deposition of gold, “dummy” SAM, gold and brush growth (full details in Supporting Information). The cantilever was double side coated with gold film to minimize the consequences of temperature drift and the effects of changes in the electrical double layer. One side was modified with hexadecanethiol (HDT) as the internal calibration. The other side was grafted with around 20 nm polymethacryloyl ethylene phosphate (PMEP) via surfaceinitiated atomic transfer radical polymerization (SI-ATRP).11 Careful calibrations were performed with cantilevers modified with inert HDT monolayer on both sides as a control experiment (Supporting Information). Non-brush-modified cantilevers exhibited negligible bending when switching between water and salt solution and solutions with different pH. The conformational changes of the brushes in response to salt solution or pH are schematically shown in Scheme 1. PMEP can Scheme 1. Schematic of Reversible Swollen/Collapse of PMEP Brush
be switched between three ionic states: fully protonated, monoprotonated/monobasic, and dipotassium salt/dibasic states, depend† ‡
The Nanoscience Centre. Department of Chemistry.
5326
9
J. AM. CHEM. SOC. 2006, 128, 5326-5327
Figure 1. Observed bending of single-side PMEP brush-modified cantilever with changing pH and schematic illustration of brush conformation in different regimes.
ing on pH.11 Figure 1 displays the bending of brush-coated cantilevers when varying the pH of the solution between 1 and 13. In region I, the brushes are fully protonated, while in region III, they are fully deprotonated, and compressive stress is generated in both strongly acidic (pH < 2) and basic (pH > 8) environments. At pH < 2, the protonated brushes are no longer soluble and will collapse, generating a compressive surface stress since the “footprint” of the polymers is too small to accommodate the collapsing chain. This is consistent with previous reports that polymer brushes generate a compressive surface stress upon polymer collapse.8 At pH > 8, the PMEP brushes are fully deprotonated, and the electrostatic repulsion between charged polymer chains leads to the development of a large compressive stress. The maximum deflection of the cantilevers, up to micrometer scale (approximate 1300 nm), is found in this fully charged state. The large increase in compressive stress upon switching from monobasic to the fully deprotonated state is consistent with a significant swelling of the brushes, as corroborated by AFM imaging (Figure S2), when switching from pH 7 to pH 9. It should be noted that the cantilever deflections are highly reversible, and that the brushes can be cycled through a number of pH cycles. The magnitude and sensitivity of the response to salt depend strongly on the length of the brushes, the grafting density, and the degree of charging of the polymer.13 We found no deflection for low (
